1. Field of the Invention
This invention relates to the field of sector servo systems for use in a magnetic disk drive, and in particular, to a phase lock loop for providing timing signals within the servo system.
2. Background Art
In a typical rotating medium based storage system, data is stored on magnetic disk in a series of concentric "tracks." These tracks are accessed by a read/write head that detects variations in the magnetic orientation of the disk surface.
The read/write head moves back and forth radially on the disk under control of a head-positioning servo mechanism so that it can be selectively positioned over a selected one of the tracks. Once in position over a track, the servo mechanism causes the head to trace a path which follows the center line of the selected track. This maximizes head-to-track registration, and permits accurate recording and reproduction of data on the track.
In order to effectively operate the head positioning servo mechanism, it is necessary that the position of the head with respect to the tracks be known. In addition to knowing which track a head is over, it is necessary to know where on that particular track the head is positioned.
In the prior art, position information is provided through the use of special servo patterns that are reproduced by the recording heads in the drive. A servo pattern is a permanent pattern prerecorded on the storage disk at the time of assembly of the completed disk drive. The servo pattern is detected by the head, and after appropriate signal processing, yields track position information. One prior art method of providing servo position information is known as a "sector" servo method.
In the "sector" servo method, bursts of servo information are disposed on a disk surface in between data areas. Each servo burst contains track position information, track radial address fields and index information. In sector servo, space-division multiplexing of data and servo information minimizes track misregistration effects, since both data and servo information are reproduced from the same surface with a common head. Thus, sector servo schemes are preferred over dedicated servo schemes for disk drives with high track density. Of course, in a dedicated servo format, the dedicated servo surface could be recorded in a sectorized fashion.
Once a data track has been located, it is important that the read/write head be kept on the center line of that track for accurate reading and writing operations. This positioning of the read/write head on the center line of a track is known as "track following." Variations from the center line of the track being followed produce a position error signal (PES) which is used to generate a corrective input to the head positioning apparatus to move the head back to the center line position.
Servo position information on either side of the center line of a data track are read and detected by the magnetic heads. A PES supplying data on the deviation of the magnetic head from the track center is detected by determining the amplitude difference of the two servo position signals. If this difference is zero, it is assumed that the head is precisely over the track. A positive or negative PES indicates that the head is off center in one direction or the other, and suitable correction signals are generated.
Prior art in the field of PES demodulation in the dedicated servo environment has typically made use of phase lock loop techniques. However, phase lock loops do not enjoy widespread application in sector servo systems. The dispersion of customer data between servo samples complicates the acquisition of lock of the phase lock loop to servo information. Also, the comparatively low servo sample rate in sector servo systems aggravates the requirement that loop lock be maintained with a minimum phase fluctuation in the event of isolated, missing servo samples caused by flaws in the record medium. It is desirable to have a phase lock loop sector servo system that can function without these difficulties.
Prior art incorporating phase lock loops in sector servo is limited in scope. U.S. Pat. No. 4,297,734 to Laishley, et al., cites application of a phase lock loop in a sector servo, but discloses no detail of its implementation. An article by Yamada et al. entitled, "Sector Servo System for High Speed Seeking and High Track Density", in IEEE Tr. Mag. Vol. 24, No. 6, November 1988, treats the design of a high performance sector servo, yet makes no mention of circuit techniques applicable to the generation of a position error signal.
One prior art method commonly used in commercially available sector servo disk files is to employ a counter which is periodically reset upon detection of a synchronization point, and advanced by a fixed frequency source. This method has the disadvantage of sensitivity to variations in disk rotation rate. It can also be mistriggered by noise. Thus, it cannot deliver a true phase-coherent reference to the data write process.